1. Technical Field
The present invention relates to an atomic oscillator, in particular, relates to an atomic oscillator that includes a gas cell, of which degradation of heating efficiency is suppressed, has high accuracy, and can be miniaturized.
2. Related Art
Atomic oscillators using alkali metals such as rubidium and cesium need to keep alkali metal atoms in a vapor state with buffer gas in a gas cell when the oscillators use energy transition of the atoms. Therefore, the oscillators operate while maintaining the gas cell, in which the atoms are sealed, at a high temperature. An operating principle of the atomic oscillators is broadly classified into a double resonance method utilizing light exciting alkali metal atoms and micro waves (refer to JP-A-10-284772, as a first example), and a method utilizing quantum interference effect (hereinafter, referred to as coherent population trapping: CPT) produced by two kinds of interfering light (refer to U.S. Pat. No. 6,806,784 B2, as a second example).
FIG. 6A schematically shows a structure of a related art atomic oscillator utilizing the CPT. An atomic oscillator 250 shown in FIG. 6A includes an optical system that is composed of a semiconductor laser 230 as a light source, a gas cell 210, and a light detector 240 as a light detecting unit, as disclosed in the second example. In the gas cell 210, alkali metal atoms (not shown) such as a rubidium atom and a cesium atom that are quantum absorbers are sealed. The semiconductor laser 230 produces two kinds of laser light (coupling light and probe light) having different wavelengths from each other and outputs the laser light to the gas cell 210. The atomic oscillator 250 detects how much laser light made incident on the gas cell 210 is absorbed by metal atom gas with the light detector 240 so as to detect atomic resonance, and allows a reference signal of a quartz crystal oscillator and the like to synchronize with the atomic resonance at a control system such as a frequency control circuit 220, obtaining an output. The light detector 240 is positioned at an opposite side of the side, at which the semiconductor laser 230 is positioned, of the gas cell 210.
FIG. 6B shows energy levels of the quantum absorbers. The energy levels of the quantum absorbers are expressed by a three-level system (Λ type level system, for example) including two ground levels (a first ground level and a second ground level) and an excitation level. When a difference between two frequencies (ω1 and ω2) of two beams, which are simultaneously radiated, of the resonance light (first resonance light and second resonance light) precisely matches an energy difference between the first ground level and the second ground level, the three-level system can be expressed by a coherent state between the first ground level and the second ground level. That is, the excitation to the excitation level is stopped.
Namely, as shown in an optical absorption spectrum of FIG. 6C, the quantum absorbers in the gas cell 210 absorb the laser light radiated from the semiconductor laser 230 and an optical absorption property (transmission) varies depending on frequency difference between the two kinds of light. When the frequency difference between the coupling light and the probe light has a specific value, neither of two kinds of the light is absorbed but transmits. This phenomenon is known as electromagnetically induced transparency (EIT) phenomenon. The CPT uses the EIT phenomenon so as to detect and use a phenomenon, in which the light absorption is stopped in the gas cell when a wavelength (wavelengths) of one of or both of the two kinds of resonance light (the first resonance light and the second resonance light) is (are) varied, as an EIT signal having a shape like δ function.
Here, when atomic concentration within the gas cell is varied in the atomic oscillator, a degree of absorption of light to the atomic gas is varied, causing an error of detection of the atomic resonance or an impossibility of detection. Therefore, atomic oscillators that are put into practical use include a heating unit for maintaining vapor of atoms within a gas cell at a constant temperature (80° C., for example) and a temperature controlling system controlling the heating unit. However, due to a demand of miniaturizing an electronic apparatus including an atomic oscillator is increased, the atomic oscillator needs to be miniaturized. Therefore, the heating unit of the gas cell is also required to be miniaturized and have a function to maintain the gas cell at a constant temperature.
In response to such demand of miniaturization, US 2006/002276 A1, as a third example, proposes an atomic oscillator having such structure that a film-like heater composed of a transparent heat element having optical transparency is provided at windows, which respectively constitute an incident surface and an emitting surface of light from a light source in an optical path, of a gas cell.
FIG. 7 shows a schematic section of an atomic oscillator (atomic frequency reference) 150 of the third example. The atomic oscillator 150 includes: a gas cell 110 in which gaseous metal atoms are sealed; a first heater 112 and a second heater 113 as heating units which heat the gas cell 110 at a predetermined temperature; a semiconductor laser 130 as a light source of exciting light exciting the metal atoms in the gas cell 110; and a light detector 140 as a light detecting unit which detects the exciting light transmitted through the gas cell 110.
The gas cell 110 is a sealed container having a cylindrical (tubular) shape. The gas cell 110 includes a cylindrical portion 101 as a first layer; a window 102 as a second layer; and a window 103 as a third layer. The window 102 and the window 103 respectively seal both ends of the cylindrical portion 101 and respectively constitute an incident surface and an emitting surface of exciting light in an optical path (shown by an arrow in the drawing). Thus a cavity T2 is formed inside the gas cell 110. Further, on respective outer surfaces of the window 102 and the window 103, the first heater 112 and the second heater 113 are provided. Incident light from the semiconductor laser 130 disposed at the outer side of the window 102 which constitutes the incident surface in the optical path in the gas cell 110 excites the metal atoms while passing through the cavity T2 in the cylindrical portion 101, and the exciting light is emitted toward the light detector 140 disposed at the outer side of the window 103 that constitutes the emitting surface. The window 102 and the window 103 respectively constituting the incident surface and the emitting surface of the exciting light are made of a material having optical transparency such as glass. Therefore, the first heater 112 and the second heater 113 respectively provided on the window 102 and the window 103 need to be made of a transparent heating material having optical transparency. As the heating material having optical transparency, a transparent electrode film made of indium tin oxide (ITO), for example, can be used. Thus the heater 112 and the heater 113 having a film-like shape are used as the heating units, enabling miniaturization of the gas cell 110 and the atomic oscillator 150 including the gas cell 110.
The third example has no description on heater wiring coupling the first heater 112 and the second heater 113 with a controlling circuit substrate including a temperature controlling circuit which controls the heaters 112 and 113. However, since the first heater 112 and the second heater 113 are independently formed respectively on the window 102 and the window 103, the heaters 112 and 113 are separately controlled. Therefore, two heater wirings are required for each of the heaters 112 and 113, that is, four heater wirings in total are required. That is, as shown in FIG. 7, the first heater 112 requires heater wirings 122a and 122b, and the second heater 113 requires heater wirings 123a and 123b. 
The heater wirings can be heat leaking paths from the respective heaters. Therefore, as the number of heater wirings is increased, heating efficiency of the gas cell may be deteriorated to increase power consumption, or temperature distribution may occur in the gas cell to deteriorate accuracy of the atomic oscillator. Therefore, the number of heater wirings of heaters provided in the gas cell should be decreased as much as possible.
Further, as the number of the heater wirings is increased, a wiring space is enlarged to make it hard to miniaturize the atomic oscillator and the controlling circuit substrate disadvantageously has a complex circuit structure.